Recently we have been successful in recovering a previously unrecognized Mycobacterium species from the diseased tissues of three patients with Crohn's disease. It is pathogenic for mice, but not for rats, guinea pigs, rabbits, or chickens. When given orally to infant goats the new Mycobacterium produced granulomatous disease of the distall small intestine in 1 to 9 months. The initial lesion consists of epithelioid cell granulomas in Peyer's patches similar to those occurring in Crohn's disease. Serologic investigations have shown that a significant proportion of Crohn's disease patients recognize antigens present in this Mycobacterium species. The proposed research will attempt to recover this new bacillus from additional patients, further explore animal pathogenicity and development of an animal model, measure immunologic responses of the agent in a population of patients, and seek to demonstrate antigen in diseased sections of intestine. Mycobacteria will be sought by bacteriologic methods, from resected tissues of patients with Crohn's disease and from controls (patients with cancer of the colon). The recovery procedures includes concentration and prolonged incubation on complex media, supplemented with a specific growth factor which is required by these organisms. Isolates will be identified by biochemical testing, lipid analysis, and numerical taxonomy. Attempts will be made to elaborate upon our recent findings in experimentally inoculated goats, and to define susceptibility in lambs, calves, and several strains of laboratory mice. Tissue tropism and the pathogenesis of early granulomas will receive particular consideration. Immunologic methods, lymphocyte blastogenesis for cell-mediated immunity, and ELISA for humoral immunity, will be used to measure immune responsiveness of patients and experimental animals to the isolated Mycobacterium sp. Peroxidase-anti-peroxidase immunohistochemistry will be employed to seek to demonstrate mycobacterial antigens in tissue sections of patients with Crohn's disease.